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Topic Review (Newest First)

02-20-2007 02:39 PM

techinspector1

Quote:

Originally Posted by xntrik

Squish will be almost nil with stock 1976 dished pistons,
even with .015 head gaskets,
except for the little rim around the outside.

Yep, I agree, I'm just trying to help this young man the best I can.

02-20-2007 01:30 PM

xntrik

Squish will be almost nil with stock 1976 dished pistons,
even with .015 head gaskets,
except for the little rim around the outside.

02-20-2007 12:40 PM

techinspector1

jj, we need to know the piston deck height also. That's the distance from the piston crown at top dead center to the flat surface of the block deck. If you don't have a 6" dial caliper, you need to buy one now. The nicest ones I've owned have been made by Starrett, Brown & Sharpe or Mitutoyo. Don't buy any cheapo junk with plastic parts. Sometimes a good value can be found on a used unit on ebay.

Anyway, you'll be able to measure outside diameter, inside diameter and depth with it. It's the depth function we're interested in here. Rotate the crank slowly back and forth through top dead center of any piston you choose (#1 is handy on the front, driver's side) until you find the piston at exact top dead center. Do this by using the slide function on the end of the caliper. Stand the caliper up on end and position the trunk of the tool on the edge of the block, then roll the slide down into the bore at the very edge of the piston right nest to the bore until you get the smallest reading. Standing at the side of the block, measure the piston at either the 3 O'clock or 9 O'clock position. Measuring at the noon or 6 O'clock position will allow the piston to rock on the wrist pin and give you an erroneous reading. You will probably find the piston down in the bore by 0.025" to 0.035" (twenty five thousandths to thirty five thousandths).

What we're doing is trying to determine the head gasket to use. Again, as I've said before, all of this stuff needs to be worked out scientifically. By knowing the piston deck height, we can determine what thickness gasket to use in order to set the squish. You'll need some squish in the motor to get it to run on pump gas with iron heads at 9.89:1 without detonating.http://search.ebay.com/search/search...2+dial+caliper

Sorry for not giving the piston info. I'm new at this, and I am learning alot. I really appreciate the advice you guys are giving me.

The pistons are stock (engine is a 1976 350). I guess they call them flattops.
They have a graduated recessed area on top which is about one half of the area of the piston top. Is that a dish piston ?

The block has not been decked. I was going to use standard head gaskets, unless you guys have a different suggestion

So with this info (and the 64cc heads), should I still go with the CompCam xe262h cam ??

02-19-2007 09:48 PM

techinspector1

Quote:

Originally Posted by jjpodlas

what if I went with 64cc cast iron heads (2.02 intake - 1.60 exhaust) ??

With the same specs we've been using, the static compression ratio would be 9.89:1
I'd be tempted to use a CompCams XE262H cam. 218*/224* @ 0.050" tappet lift, 0.462"/0.469" theoretical valve lift, 110* lobe separation angle. Makes power 1,300 to 5,600. I'd use a 3.50-3.70 gear and a 1,800-2,200 rpm stall converter and a good set of equal-length headers into 2 1/2" pipes to the back with X or H crossover just behind the collectors and free-flowing mufflers.

Again, call your favorite cam grinder for an exact recommendation.

Other board members may have slightly different advice for you, but I'm just trying to get you close so you can see generally what cam to use with what compression ratio. It's a scientific process. You can't just pick a cam out of mid air because you read some hype about it or your buddy suggested it.

baddbob has asked you twice now to describe your pistons. Without specific information, what I've suggested here is just so much doo-doo.

02-19-2007 09:41 AM

baddbob

What pistons can make a huge difference on compression ratio, example- with 64cc heads some flattops will be only 9.3-1 while others will be well over 10-1. Knowing the static compression ratio will help determine how small you can go on the cam with pump gas.

02-19-2007 07:53 AM

jjpodlas

cam question

what if I went with 64cc cast iron heads (2.02 intake - 1.60 exhaust) ??

02-19-2007 01:17 AM

JeffB

SBC Cam

I think you've gotten some good advice a dual pattern cam is what will fill the bill for you .You may want to check my adds I am selling a lot of NEW SBC parts cause I sold the Chevy project I had and went Ford,PM me and I can give you a list and prices.

02-19-2007 12:42 AM

baddbob

what exact pistons are in the engine? Has the block been decked at all? What headgaskets are you going to use?

If you really are at 10-1 you'll need a 270ish cam to make it work on the street but power is going to stop climbing at 5500 rpm with those heads.

It's doubtful you're gonna make 300 horsepower with this combination though. It would just be a real nice daily driver type motor. Using 1.6 rockers on both intake and exhaust would yield a theoretical valve lift of 0.469". This cam is ground on a lobe separation angle of 110*. If you wanted a little more rump at idle, you could have it ground on 106* angle. It's a single pattern cam. You may want to look around for a dual pattern cam if you think it would help the exhaust side of the motor with stock heads. I'd be just as happy with a single pattern with this particular combination. Here's a dual pattern cam that is ground on 112* angle just so you can compare cams.http://www.cranecams.com/?show=brows...tType=camshaft

Do not cut a check for a cam based on the information I've provided here. ALWAYS CALL YOUR FAVORITE CAM GRINDER FOR A DEFINITE RECOMMENDATION. I'm just trying to give you an idea of cams that would work with the info you've provided. A narrower angle such as 106* will give you a rump-rump idle and build good bottom end, but will peak quicker and lay down on the top end. A narrow angle may not provide enough manifold vacuum to operate power brakes properly. A wider angle such as 112* will give a smooth idle and excellent manifold vacuum for power brakes and such and will be stronger on the top end without giving up much on the bottom.

02-18-2007 04:22 PM

jjpodlas

cam question

what if I went with 76cc cast iron heads with 1.94 intake & 1.50 exhaust. Then what cam should I use in terms of lift, duration, etc

02-18-2007 02:12 PM

techinspector1

jj, is it stock bore with stock-type Chevy 4-eyebrow pistons?
Do you know how far down in the bore the piston is at TDC?

If we assume a stock short block with a piston deck height of 0.030", 7 cc's in the piston crown, 58 cc heads and 3.5 cc's in a 4.100" x 0.016" shim gasket, the static compression ratio will be 10.61:1.

Like Curtis73 said, the cam needed at this level of compression will be way beyond the capabilities of the heads. You'd have a very soggy bottom end due to the duration and intake closing point required to prevent detonation, plus the motor will run out of breath just as the motor starts to come up on the cam.

This looks like a very bad idea. To make the heads work, lower the compression with dished pistons down to around 9.0:1 and use a short cam that makes power from 1,000 to 4,500.

02-18-2007 01:06 PM

curtis73

The cam you'll need at 10:1 is probably WAAAAY beyond what those heads can flow. 300 hp can be had with a stock cam, vortec heads, and 9.5:1. I'm doing it right now in a 350. In a nutshell, you have a big mismatch with those heads' flow and compression for a mild 300 hp combo

How about a head casting number? Sounds like 305 heads to me; small chambers, little valves.